Cavitating jet generating devices have been known and documented in the technical literature. A number of patents have also issued pertaining to various cavitating jet systems, U.S. Pat. Nos. 5,217,163; 5,154,347; 5,125,582; and 5,086,974 are examples of technologies already known in the art. Cavitation has been known as a deleterious factor, for example, in the marine industry where it may severely damage propellers and other underwater components of ships. When cavitation bubbles collapse on a surface, the collapsing bubbles produce very high-speed micro jets which are responsible for the damage to the surface by erosion. The same erosive power of cavitation is beneficial when used in certain applications, for example fragmenting ore-bearing hard rocks in the mining industry or removing solid particles from a substrate, to name but a few possible applications.
Known cavitating waterjet systems only produce effective high-speed cavitating jets when submerged. When a high-speed continuous waterjet is fully submerged in quiescent water, shear layers develop in the mixing zone of the jet and the still water. These shear layers produce vortices which give rise to cavitation bubbles (containing water vapor, not air) in the high-speed waterjet. Prior-art cavitating jets in air usually experience a loss of cavitating power due to a partial collapse of the vapor bubbles present in the cavitating jet after it leaves the nozzle. Because so much power is lost before it reaches the target object, the cavitating action on the surface of the target object is undesirably low.
High-pressure non-cavitating jets can be used in the applications mentioned above (fragmenting, surface cleaning, etc.); it is known, however, that a cavitating jet can achieve the same erosive effect as a non-cavitating jet using considerably less pressure and hydraulic power. Therefore, employing cavitating jets can not only reduce the costs but also enhance operational safety.
It is possible to generate a cavitating jet in air by artificially submerging a continuous waterjet (R. Houlston and G. W Vickers, Surface Cleaning Using Water-Jet Cavitation and Droplet Erosion. Proc. 4th Int. Symp. on Jet Cutting Technology, 1978, paper H1, pp. H1-1/H1-18). However, this system is relatively complex as it necessitates two separate sources of fluid.
Applicant published a study of a nozzle device for generating cavitating or pulsed water jets (M. M. Vijay, R. J. Puchala and N. Paquette, Study of a Novel Device for Generating Cavitating and Pulsed Water Jets. Proc, 13th International Conference on Jetting Technology, Sardinia, Italy, 29-31 Oct. 1996, BHR Group Limited). The elementary reverse-flow nozzle was also disclosed in a further publication (M. M. Vijay, C. Bai, W. Yan and A. Tieu, “Reverse flow nozzle generating natural cavitating or pulsed water-jets—Basic Study and Applications”, Jetting Technology. pp. 243-252, BHR Group (2000). The reverse-flow nozzles disclosed in these publications utilized a continuous jet inlet and distinct lateral inlets for the reverse jet and the shroud jet. The nozzle design was thus complex as it required separate lateral inlets for the reverse jet and shroud jet. A simpler, more efficient, more practical and more cost-effective reverse-flow nozzle thus remained highly desirable. A solution to this technical problem is disclosed herein.